Z axis problem

Well, I have my Prusa Mendel. Completed construction, everything went smoothly untill I tried to home the Z axis.
The nozzle is going up, instead of going down. Thing is, when I send a command to go up a few centimeters, it goes up. What could it be? All connections are good, I cannot understand why it has this strange behaviour, it keeps searching for the 0 position up.

More news:
I tried switching the Z motor wires: when I tell the Z axis to go +10mm, it goes down. However, it still searches for 0 in the up direction. I cannot understand how is this possible, why is searching for the 0 point upstairs, even the wires are inversed? I completly blew my mind, I am totally in the fog now...

so switch the wires back obviously

Would help if you tell us what firmware you are using, how many endstops. mechanical or optical etc etc.

Don't worry too much chances are its simply a setting in your firmware.

good luck

I agree, it might be something very small, but right now I feel like a behedead chicken. I don't know where to find the firmware, or the tools necessary to read/modify the firmware and I keep reading forums/helps/tutorials...
I have the latest Reprappro Prusa Mendel, this year's edition. I bought it from reprappro, and I have 3 mechanical endstops.

Ok slow down ...dont get frustrated.

Im having a quick look at the reprappro website and documentation
now sure what firmware you are using ( or is loaded as standard)

Maybe you have wired the Z end stop wrong.
It should be wired to the NC (normally closed contact) In other words the circuit is complete normally and when the end stop os activated it breaks the circuit ...just an idea and without more info tricky to tell.

Im assuming you read an tried this ( from your manual )
Motors going backwards
RepRap works with righthanded
Cartesian coordinates relative to the bed not the frame. That is to say that
looking down on the bed from the front of the machine X runs from left to right, and Y runs from front to
back (like a graph). Z runs up towards the top of the machine.
Remember that it is the movement of the printing head that counts: when Y increases, the bed will move
towards you.
If you find that an axis is backwards, it is simple to reverse its motor: just power down and then reverse the
order of its wires into the controller, so, for example, [black, green, blue, red] goes to [red, blue, green,
black]. Don't forget to turn the power off before disconnecting and connecting wires.
Endstops
The endstops are only checked during homing. To test them, send a homing command for the X, Y and Z
axes in turn, for example
G28 X0
As soon as you press Send and the axis begins to move, activate the relevant limit switch to halt movement
of the axis. If activating the switch does not halt your axis, check your wiring (Huxley here or Mendel
here).
Homing
You are almost ready to home your machine. Before doing so, ensure the Z endstop is high enough on the
Z smooth rod to trigger the switch without the head ploughing into your heatbed.
Press the HOME ALL button and your machine will find its reference position at X0 Y0 Z0.
(If the machine stops before reaching an endstop it may be because the endstop is being falsely triggered
(http://forums.reprap.org/read.php?214,133203) by interference from the stepper motor wires.)

Thanks for the quick info.
Yup, read it very closely. And my problem is not there, the printer keeps searching for its Z0 (Z home point) upwards, instead of downwards.
More info:
If I tell it to go up, it goes up. If I tell it to go up 10 cm, it goes 10 cm. Go down 10 cm, it goes. But it won't go downward from this point, no matter what (so I assume it considers that starting point the Z0).

OK
from what ive seen on the website your using marlin firmware

I kind of recognise the issue in as much as i remember seeing something similar a while back when i was configuring a new controller board for somebody
It does sound like its looking for a home point
so just to confirm it will move down but only once and only 10mm ( i think you mean mm NOT cm! )

So first off will it home to X and Y ?? If so when it does, does it move to home then when it touches move back slightly then move to home again and stop ??

i think your motor is the moving the wrong way. WIth mine when i first switch on it will home ( irrespective of where the axis ( X,Y or Z ) is) but it will only move 10mm away from home ( on any axis) until it has seen the home signal once . This makes sense as the firmware has no idea where the axis are until the respective home signal has been "seen" The firmware contains limitations for your build platform ( X& Y) and machine max build height (Z). SO it doesn't want to drive the steppers beyond those points and the only way it knows where those points are is when it has seen each respective once since power on.

just checked on my machine that uses marlin as well

yours appears to be acting the same way as mine.
from boot up it will only allow 10 mm in the z axis home direction if i use the Z axis control in pronter face so i move it once towards home then it wont move any more. then i click the Z home button and it move home till the home switch is activated

Exactly!
When it homes, it first looks for X and Y, then Z. And when it hits the endstop, it will hit it again, just slightly (to confirm?).
But how can be the motor? It executes the commands with no problem (moving in the directions clicked in pronterface). I read and read (hope my eyes won't need more glasses after 9 hours of non stop reading) and from what I understand, I might need to modify the direction in wich the firmware looks for the Z0 endstop. So, is this done in the firmware? Is it a command in pronterface?

Found something interesting here: [forums.reprap.org]
Now I have to find out how I have to do those changes.

I also found this: [forums.reprap.org]
Veeeeeery sad... Hopefully I won't end up like this.

Edited 3 time(s). Last edit at 05/20/2013 06:58AM by imprimare3d.

Nah dont give up ...there is always a solution ...its just a question of patience.
The issue here is we don't know what exactly is installed on your control board.
when you press connect in pronter face what does it reply with when it connects ?? ( the text will appear in the pronterface window)

Yes the endstops are configured in the firmware config files

Im afraid i have never used your kind of board so im working blind but its worth reloading the firmware. Contact your supplier and confirm which firmware you have and which software is needed to upload firmware.

I have these on my computer: [github.com]
[reprappro.com]
I am searching throught them, maybe there is a work around to this problem...

Edited 1 time(s). Last edit at 05/20/2013 07:29AM by imprimare3d.

Hi, not sure if this has already been solved, but I had similar issue and it turned out that my z-axis endstop wiring came lose. Checking with your multimeter if everything is still well connected is aways a good idea

I suspect that a thred that's been dead for about 6 months can be considered solved. That's a *long* time on the internet. Even more so when all the posts in the thread happened over the course of < 48 hours.